Direct-fired unit heater



y 1945- D. HENDERSON ET AL 2,376,140

DIRECT FIRED UNIT HEATER Filed Jan. 5, 1942 5 Sheets-Sheet 1 y 1945- D. HENDERSON ET AL 2,375,140

DIRECT FIRED UNIT HEATER Filed Jan. 5, 1942 s Sheets-Sheet 2 MW ifzgwwm 7 alt/Wi k WMW y 1945- v D. HENDERSON ET AL 2,376,140

DIRECT FIRED UNIT HEATER Filed Jan. 5, 1942 5 Sheets-Sheet 5 l VENTORS M010 .57 WW and fl ATTORNEY-S Patented May 15, 1945 DIRECT-FIRED UNIT HEATER David Henderson, Ralph M. Rush, and Harold C.

Anderson,

Pennsylvania.

Pittsburgh, Pa;, asslgnors to Dravo Corporation, Pittsburgh, Pa.,

a corporation of Application January 5, 1942, Serial No. 425,582

. trating the invention. in application to a heater Claims.

This invention relates to heaters designed and intended primarily for use in industrial plants and of the type that has come to be known as the direct-fired unit heater. This heater is a double-walled structure. Heat is generated within the inner chamber by the burning of suitable fuel (ordinarily gas or oil), and through the walls of the inner chamber heat is transmitted to air that flows in outspread stream between the walls. Heaters of this type become economically serviceable where steam is not available; and they are chiefly used in industrial plants, in public garages, hangars, and the like. They may, of course, be used in whatever place they are found suitable. The requisites are that the structure be adaptable either to suspension from above or support from beneath; that it occupy comparatively small floor area; that it be capable of firing at good efflciency, whether the fuel be oil or as; and that it be dependable and safe in use. A heater of the type in, view is shown and described in Letters Patent No. 2,115,057, granted April 26, 1938, on the application of Harold C. Anderson, one of the present applicants. The present invention consists in. refinements in structure, in consequence of which heating elliciency is increased; durability is increased; and the space occupied is diminished.

In the accompanying drawings Fig. I is a view in side elevation of a gas-fired heater of the invention, and Fig. II is a view of the same in end elevation. In both of these two views portions of the walls of the structure are broken away, and internal parts are shown in elevation and in section. Fig. III is a fragmentary view of the heater, as seen in horizontal section, on the plane III-III of Fig. vI. Fig. IV is a view in transverse section, on the plane IV-IV of Fig. III. Fig. V is a fragmentary view, illustrating in perspective the manner in which the side walls of the inner chamber of the heater, formed of corrugated sheets or metal, are assembled edge to edge and ready to be united, by welding to a metal angle bar inserted between the edges. The angle bar is shown iragmentarily in position to be.

advanced length-wise between the meeting edges of the corrugated sheets. Fig. VI is a view to larger scale and in transverse section, and shows the assembled and united corrugated sheets and angle bar. Fig. VII is a view of the same in side elevation. Fig. VIII is a view corresponding to Fig. VII, and shows the same assembly after the webs of the angle bar have been trimmed to the contour of the edges of the corrugated sheets. Fig. IX is a view of like nature with Fig. I, illusadapted to be hung from above, as from a roof or ceiling, and to deliver its stream of heated air downwardly.

Referring first to the heater shown in Figs. I-IV, this is a heater that is designed to stand upon a floor. It is a double-wall structure. The inner wall is formed essentially of two corrugated sheets I of heavy-gauge steel, shaped to afford a heating chamber of cymbiform or stream-lined cross-section, as clearly shown in Figs. II and IV. This heating chamber is arranged in the horizontal position shown in the drawings, with the angles of its cymbiform shape disposed in a common vertical plane.

The seams of union of the steel sheets of the heating chamber may advantageously be formed in the manner illustrated in Figs. V-VIII. The sheets are cut off square, or approximately so; two lengths of rolled angle 2 are provided (above and below); and to their flanges the sheets at their cut-oil edges are welded. Advantageously, after the union of the sheets with the angles has been effected, the portions of the flanges of the angles that extend across the troughs of the corrugations are cut away (cf. Figs. VII and VIII), leaving the troughs unobstructed. The heating chamber is closed by heads 3 of sheet steel, completing a gas-tight welded steel chamber. This manner of uniting the meeting edges of the corrugated sheets is more specifically described in copending application Serial No. 581,062, filed March 5, 1945, by Ralph M. Rush.

The outer wall or casing is made of heavy sheet steel. As shown in Fig. I, three panels welded together form the overarching sides 4 of the casing and at its edges this overarching structure may be separably secured in air-tight union with ends 5 and with a base. At its peak this structure carries the air-delivering hoods B. This overarching structure 4 may be released and removed, that access may be had to the interior.

The casing rises from a base 1. Upon this base the ends 5 of the casing are permanently mounted, and to it (as to the ends 5) the overaching structure 4 is removably secured. In the base are formed air intakes 8; and within the base are arranged fans 9, driven by a motor Ill. The fans draw air through the intakes and drive it in an upward flowing stream through the space at which the outer casing stands apart from the walls of the heating chamber, and out through the delivery hoods.

It will be perceived that the heating chamber arranged within the casing causes the fan-impelled stream of air to divide into two branches of small depth, outspread over the surfaces of the heating chamber; and allows the branches to reunite before emerging through the hoods 6. The walls of the heating chamber are equipped exteriorly with deflector plates H, such as those shown and described in the Anderson patent named above, for directing the air in its flow, that it shall more effectively take up heat from the walls. The upward-flowing stream envelops also the end Walls of the heating chamber.

Through one of the ends of the outer casing and through the adjacent head 3 of the heating chamber extends a burner structure l2. Of this burner structure it suffices to sa that it is adapted to project longitudinally into the heating chamber its flame. In this instance it may be understood to be a flame of burning gas. Within the outer casing and external to the heating chamber and on the upstream side of the heating chamber (relatively to the direction of the flow of the air stream) is arranged abatter of tubes, ordinarily of steel, of composition well suited to heat-exchange. The tubes are here shown to be eight in number, symmetrically arranged with respect to the vertical and longitudinal mid plane of the heater. To these tubes at one end the heating chamber opens, and from them at the opposite end a suction fan I! draws the spent gases.

The invention lies in refinements in shape and structure of a heater having the general characteristics that have now been indicated. And. first, we shall describe the lining I5 within the heating chamber.

The Anderson patent named above shows a trough-like fire-box of insulating brick masonry, built within and in the lower part of the heating chamber. A b'urner arranged in an opening in the head of the heating chamber projects its flame longitudinally into this fire-box. Between the exterior surface of the fire-box and the interior surface of the heating-chamber walls narrow passageways are formed, and through these passageways the burnt gases, leaving the heating chamber, are drawn downward by fan suction. Anderson's fire-box then serves triple purposes: it serves, in the first place, as a heat-absorbing, heat-radiating body, adapted to promote and intensify the combustion of the fuel; consistin of insulating fire brick, it prevents the travel of heat from the intensely hot inner surface of the fire box to the cooling gases on the outside; and it serves, in the third place, to define passa eways for the hot gases and to confine the out-going stream of hot gases, so that they shall flow over the inner surface of the heating-chamber wall in the re ion where the advancing stream of air heins its progress over the outer surface of the heating chamber wall. In the heater of this invention outflow passa eways between flreb x and heating-chamber wall are done awav with. Instead. the outgoing gases pass from the heating chamber through the battery of tubes I3. It is over the outer surfaces of these tubes that the advancing stream of air flows. It is to these tubes that the outgoing gases now make their ultim te delivery of heat; and it is from them that theincomin air takes up its initial heat increment. And. since it no longer is necessary to provide passageways between fire-pot and heating-chamber wall, we find relatively heavy and self-sustaining masonry to be no longer necessary, nor need it be of insulating brick. Instead, we provide an adequate heat-absorbing and heat-radiating body by shaping plastic refractory as a lining immediately upon the inner surface of the corrugated walls of the heating chamber. This lining hardens in situ. It is a A continuous lining and, with the qualification presently to be noted, extends in continuity upon the inner surface of the heating-chamber walls throughout the lower portion ofthe chamber. It promotes combustion; it protects the heatingchamber walls where the flame is most intense; and it both conducts and radiates heat to the metal walls I that underlie it and arch above it, that they in turn may deliver heat to the air streaming over their outer surfaces. 1.) It is not necessary, nor even desirable, to form the lining l5 of a refractory that is of high heatinsulating quality. Its primary purposes are to promote combustion of the incoming stream of fuel, and to protect the metal walls of the heating chamber where the flame is most intense. Inevitably the lining radiates heat upwardly to h u e per walls of the heating chamber; and inevitably the refractory lining is in appreciable degree heat-conducting, so that heat penetrates the wall of the heating chamber, even where the wall carries such internal lining. A relativel thin lining of refractory of relatively low heat-insulating effect sumces for the primary purposes indicated. Beyond those, heat penetration through the refractory-faced wall is not undesirablerjt is desirable, rather. In point of fact the lining that we use with entire satisfaction in service is of a material that in heat conductivity exceeds the flre brick of the older heater alluded to, three times over; and is of a thickness that is one third that of the Walls of the old fire-pot.

Such a lining then may be provided, and will be found to be in many respects superior to the fire-box of masonry of the earlier patent named.

In the matter of dimensions of the heatin chamber, the net result of the change that has been described is that a heating chamber 2'7 inches wide will give an output of heated air equivalent to that of a chamber 39 inches wide built according to the earlier design. This means that the overall width of the entire unit may be reduced by 12 inches, and brought within a floorspace no greater than that of a steam-heating unit affording equivalent hot-air output and involving equivalent electricmowereqnsumption.

This change in the structure of the fire-box effects a saving, not in bulk only, but in weight also, and in cost. In point of fact, the saving in cost is as great as the expense of the added preheating tubes (the tubes 13) so that, in spite of increased labor and material costs, it is possible to market the new heater at approximately the same price as the old, and to afford the customer a superior article.

Another great advantage that follows upon the use of the plastic firebrick lining for the heating chamber is that the lining may be applied in the fabrication of the heater; and the heater may be shipped with the lining already in place.

In operation a fire-box formed as a plastic refractory lining of relatively light weight has the further advantage that the quantity of heat "0 stored within it is less. Indeed, in the old heater,

so great was the quantity of heat stored in the fire-box that when after service the fuel burner was shut down, it was necessary still to run the cooling fans for a long time; and this with cor- 0 responding waste of fan-driving energy. In the new heater the fans and the burner may be put chamber is, as has been said, prolonged to sharp edges both below and above (Figs II and IV); and it has been found that, even with the addition of the tubes I3, a H. P. motor will afford a hot-air delivery such as in the old heater required a 7 /2 H. P. motor. And in this respect for the first time the floor-mounted direct-fired heater may be operated with electric-power consumption reduced and made fairly comparable with that of a steam heating unit of equal capacity. I

Because of the stream-lined tapering of the upper edge of the heating chamber 9. defect inherent in the old heater is corrected: the defect, namely, of overheating at that point. The development of a hot spot there is in the improved structure prevented both by the withdrawal of this portion of the structure (the peak of the heating chamber) to a greater distance from the flame, and also by the more, effective sweep of the stream of air over every inch-of the exterior surface of the chamber wall.

The preheater tubes I3 arranged externally of the heating chamber are more effective than the passageways in the old heater because throughout all their circumferential extent the walls of the passageways formed by these tubes are heatexchanging walls.

Preferably spiraled plates (called twisters) are arranged within the tubes (a known device), to cause the stream of hot gas within the tube to make more effective sweep over the tube surfaces.

The tubes, of course, may be made readily accessible for cleaning.

A further refinement of shape and structure is found in the provision for the eduction of the stream of gases from the heating chamber. Comparison of Figs. I and III will show that the gases pass from the heating chamber to the tubes I3 (here shown to be eight in number) through a plurality of exits formed in the chamber wall arranged in a widely spread group near the burner end of the chamber. In order to accomplish this. the lining I5 of refractory, shaped while in plastic condition, is at this end of the heating chamber tapered inwardly toward the fuel burner; it rises from the heating-chamber wall; and this tapered rising portion I50 of the refractory lining forms with the heating-chamber wall a, space I5I. From this space the exits to the tubes are arranged and the escaping gases stream into this space to gain access to the tubes.

It is advantageous to provide for the exit of the hot gases from the heating chamber downward through the side walls to the tubes I3, as shown in Fig. I, rather than through the end wall or head of the heating chamber, and this for several reasons. For one, it will be seen in Fig. I that there is space on the end wall 5 to the right, and beneath the burner l2 upon which the switch panel for the control of the motor may be placed. In unit-heaters of other design space is lacking upon the end wall, and it has been necessary to mount the switch panel upon the side wall of the casing. The placing of the switch panel upon the side wall becomes in particular cases a matter of embarrassment, because the side upon which the panel is borne may be the side that in installation faces the wall of the building. Furthermore, it is advantageous that the panel be on the end wall both because the electrical connections are shorter and in out-ofthe-way position, and because the over-all width of the structure is in no case increased thereby.

Another advantage will be perceived in that a single elbow in the tubes I3 sufiices to bring the outflow of the gases into line for delivery through the opposite left-hand head of the unit. Were the exit arranged in the right-hand head of the heating chamber, it would be necessary for the gases to make three right angle turns before entering the tubes.

Another advantage of arranging the exit of hot gases to the tubes as shown in Fig. I, rather than, as suggested. 'through the end wall of the heating chamber, is that the space between the end wall 3 of the heating chamber and the outer casing 5, is at this right-hand end left for free and unobstructed flow of air from the fans below.

Yet another advantage is this, that the tubes are contained wholly within the casing. The gases as they leave the heating chamber are ven hot; and if at this point the gas-containing walls were exposed. persons accidentally touching them might be uncomfortably or severely burned. Such accidents are, in this structure. prevented.

The spreading of the exit in a number of separate openings through the heating-chamber wall (in this instance eight) has effect in avoiding concentration of heat and the development of a hot spot in the wall of the heating chamber at the place where the gases emerge from it.

Within the heating chamber and near the end remote from the burner I2 a wall I10 of masonry stands athwart the line of flame flow. This wall may be formed in any preferred manner. Conveniently it is a built-in wall of firebrick. As best seen in Fig. III it is of angular cross-section, symmetrically placed. with the peak of the angle arranged in the vertical mid plane of the structure, and re-entrant with respect to the right-to-left advance of the flame. The angle maybe of the order of In the gas-fired heater of Figs. I-IV. the burner l2 projects the flame centrally and longitudinally within the fire-box, or refractory-lined lower portion of the heating chamber: the stream of flame impinging on wall I10 turns upward in its course. returns in the upper portion of the heating chamber, and, spreading laterally and bifurcated, descends (on the two sides of the entering line of flow) to the space formed between the tapered end of the liningand the metal walls of the heating chamber cf. Figs. I and III); from this space the hot gases find exit through the tubes I3. The course of the stream of flame is indicated by arrows in Fig. I. In actual operation the spent gases pass from the tubes I3 at a temperature that ranges from 400 to 500 F. That is to say, the temperature always is above the dew-point, and corrosion does not take place.

It will be noted that, in the operation of the heater of Fig. I. the stream of flame and hot products of combustion is reversed within the heating chamber, traversing the length of the chamber twice before finding exit at the intake end to the tubes that serve to preheat the oncoming air; and that, in consequence, the efflciency of the areas of heat-exchange through the chamber walls is increased.

In Fig. IX a heater of the invention is shown, arranged for suspension from above, as from the roof structure of a factory building. The air-impelling fans l9 are arranged above the heating chamber and the delivery hoods .20 are arranged below. The flow of air is vertically downward. The pre-heater tubes 2| are arranged in the air stream on the up-stream side of the heating chamber 22. The fire-pot that consists of a thin lining 23 of refractory, shaped while in plastic condition in situ, is arranged as in the heaters of Figs. I and V in the lower part of the heating'chamber, and extends from end to end of the heating chamber. As here shown the heater is provided with a burner for oil as fuel; the wall 24 rises within the fire-pot near the end of the heating chamber opposite that penetrated by the burner block 25. In this case a the bridge wall is of angular horizontal section, with the angle re-entrant toward the advancing flame; and the pre-heater tubes 2| open through the side walls of the cymbiform heating chamber, in the upper part and near that end from which the flame is projected. The course of the stream of flame is indicated by arrows.

Fig. IX serves additionally to show that, in place of one or all of the air-delivery hoods 20, a conductor 26 may be provided, to carry the heated air to any desired place of delivery.

While we have indicated that the heater of Fig. I is equipped with a gas burner; and the heaters of Fig. IX with an oil burner; we wish to make plain the fact that the heater in any of these forms may be equipped with a suitable burner and fired with any desired fuel.

Operation is in familiar manner: the stream of flame has its source in the burner introduced through one of the heads of the heating chamber, it circulates through the heating chamber, combustion is completed, and the hot gases stream through the preheating tubes under the suction of a fan. The walls of the heating chamber and of the tubes are heated, and the stream of air flowing over these hot surfaces takes up the heat and is delivered in the desired heated condition. As has been explained, local concentration of heat is avoided and a more serviceable heater is produced.

We claim as our invention:

1. In a unit heater suitable for industrial plants and other buildings that includes a casing, means for maintaining a stream of air flowing in vertical course through the casing, an elongate chamber of uniform cross-section throughout its extent and formed of heat-conducting material arranged horizontally within the casing and extending athwart the path of such air stream, a

battery of tubes of heat-conducting material arranged within the casing and on the up-stream side of said chamber and extending'athwart the path of such air stream, the tubes being at one end in communication with said chamber, and means for projecting within the lower portion of said chamber and in the direction of its horizontal extent and thence into and through said tubes a stream of burning fuel and the products of combustion, the invention herein described which consists in a lining for said chamber in the form of a trough of refractory material tapered at one end elsewhere lying in contact with the chamber wall and at its tapered end defining spaces at one end of the chamber at each side of the fuel burner, and a plurality of openings through the chamber wall directly connecting said spaces with the tubes of said battery.

2. In a unit heater suitable for industrial plants and other buildings that includes a casing, means for maintaining a stream of air flowing in vertical course through the casing, an elongate chamber of uniform cross-section throughout its extent and formed of heat-conducting material arranged horizontally within the casing and extending athwart the path of such air stream, a fire-pot of refractory material arranged within said chamber and in the lower part thereof, means for projecting a stream of flame within such fire-pot and in the direction of the horizontal extent of the chamber, and means for conducting the hot gases from the chamber, the invention herein described which consists in arranging within the flre-pot and adjacent the end toward which the stream of flame is projected a transverse wall of angular horizontal section arranged with its angle converging in the direction of advance of the projected flame, and the means for conducting the hot gases from the chamber adjacent the end from which the stream of flame is projected.

3. In a unit heater for industrial plants and other buildings that includes a casing, means for maintaining a stream of air flowing through the casing, a chamber with side walls of heat-conducting material arranged withinthe casing and extending athwart the path of such air stream, a battery of tubes of heat-conducting material arranged within the casing on the up-stream side of said chamber and also extending athwart the path of the air stream, the said tubes at one end being in communication with said chamber, and means for causing a stream of flame and hot gases to flow in continuous course through said chamber and thence through said tubes, the invention herein described which consists in arranging a burner centrally in one of the end walls of said chamber and arranging the openings of communication from said chamber to said tubes through the side walls of the heating chamber and adjacent the end of the heating chamber in which the burner is arranged, and in groups on the two sides of the mid plane of the chamber and symmetrical with respect to such mid plane, whereby the flame flow from the burner is projected centrally and longitudinally through the heating chamber, and is caused to return again in opposite course and in its return to spread into two branches that pass laterally on the two sides of the entering flame as they approach the openings to the tubes.

4. In a unit heater for industrial plants and I other buildings that includes a casing, means for maintaining a stream of air flowing vertically through said casing, a heating chamber with sidewalls of heat-conducting material arranged in position of horizontal extent within the casing and athwart the path of such air stream, a battery of tubes of heat-conducting material in communication with said chamber arranged within the casing on the upstream side of said casing and also extending athwart the path of the air stream, and means for causing a stream of flame and hot gases to flow through said chamber and thence in continuous course through said tubes, the invention herein described which consists in giving to the heating chamber cymbiform shape in cross-section and arranging it with its edges in common vertical plane, and combining with the so shaped and arranged heating chamber a burner arranged in an end wall of the heating chamber and centered in the vertical plane last mentioned, a trough-like fire-box arranged in the lower portion of the heating chamber conforming throughout a portion of its extent to and resting upon the side walls of the chamber and tapering at one end to the burner, such tapered portion of the trough forming with the side walls a space within the heating chamber open above, the openings from the heating chamberto the tubes being in direct communication with such space and being formed in two groups on the two sides of the said vertical plane,

5. In a unit heateribrindustrial plants and 'other buildings that includes a casing, means for maintaining a stream of air flowing vertically through the casing, a chamber with side walls of heat-conducting material arranged in position of horizontal extent within the casing and athwart the path of such air stream, a fire-pot arranged within the heating chamber and in the lower part i thereof, a battery of tubes of heat-conducting material arranged within the casing on the upstream side of said chamber and also extending athwart the path of the air stream, the said tubes at one end being in communication with said chamber, and means for causing a stream of flame and hot gases to flow in continuous course through said chamber and thence through said tubes, the invention herein described which consists in the arrangement of the air intake above and the air delivery below whereby the flow of air is vertically downward within the casing, in the arrangement of, a burner in one of the end walls of the heating chamber, and in the arrangement of the openings of communication from the heating chamber to the tubes through the upper walls of the heating chamber adjacent the end in which the burner is arranged, whereby the flow of flame and gas from the burner, projected longitudinally in the lower part of the heating chamber and within the fire-pot, is caused to return again in the upper part of the heating chamber to the exit.

6. Apparatus of the class described comprising an internally fired horizontally extending combustion chamber having a heat-conducting wall, said chamber having in its lower portion an internal lining of refractory material that forms an elongate trough-shaped fire box, the

heat-conducting walls of said chamber above the elongate fire-box being exposed to the immediate efiect of heat generated in the fire-box, an outlet for waste gases, a horizontal bank of heatexchange tubes, said tubes communicating at one end with said combustion chamber at points 10- cated externally of said trough-shaped fire-box, and at opposite end with said outlet, means for firing said fire box and effecting the sweep of the flame and hot products of combustion through a succession of horizontal courses of flow, one of which extends in horizontal direction in said fire box, another of which extends in horizontal direction above said fire box and in contact with the heat-conducting combustion chamber walls above said fire box, and another of which extends through the tubes of said bank, and means for effecting the movement of air over and in intimate contact with the external surfaces of said combustion chamber and tubes in a direction substantially perpendicular to said horizontal courses of flow of the flame and products of combustion.

'7. Apparatus of the class described comprising an internally fired horizontally extending combustion chamber having heat-conducting side walls, an outer housing having side walls spaced from the side walls of said chamber, said chamber having in its lower portion an internal lining of refractory material that forms an elongate trough-shaped fire box, the heat-conducting walls of said chamber above the elongate firebox being exposed to the immediate effect of heat generated in the fire box, an outlet for waste gases, a plurality of horizontally extending heatexchange tubes, said tubes communicating at one end with said combustion chamber at points cated externally of said trough-shaped fire box,

and at opposite end with said outlet, means for firing said fire box and effecting the sweep of the flame and hot products of combustion through a succession of horizontal courses of flow, one of which extends in horizontal direction in said fire box, another of which extends in horizontal direction above said fire box and in contact with the heat-conducting combustion chamber walls above said fire box, and another of which extends through the tubes of said bank, and means for effecting the movement of air, in a direction perpendicular to said horizontal courses of flow of the flame and products of combustion, over and in intimate contact with the external surfaces of the said tubes and through the spaces formed by and between the side walls of said combustion chamber and said outer housing.

8. Apparatus of the class described comprising an internally fired horizontally extending combustion chamber having heat-conducting walls, said chamber having in its lower portion a firebox open from above, with the heat-conducting walls of said chamber above the fire-box exposed to the immediate effect of heat generated in the fire-box, an outlet for waste gases, a horizontal bank of heat-exchange tubes communicating at one end with said combustion chamber at points located externally of said fire-box and at-opposite end with said outlet, means for firing said fire-box and effecting the sweep of the flame and hot products of combustion through a. succession of horizontal courses of flow, one of which extends in horizontal direction in said fire-box,

' another of which extends in horizontal direction above said fire-box and in contact with the heatconducting walls of the combustion chamber above said fire-box, and another of which extends through the tubes of said bank, and means for effecting the movement of air over and in intimate contact with the external surfaces of said combustion chamber and tubes in a direction substantially perpendicular to said horizontal course of flow of the fiame and products of combustion.

9. Apparatus of the class described comprising a combustion chamber having a bottom and sides, with the sides formed of heat-conducting material and converging inwardly to form an overarched top portion, walls spaced from the exterior of said chamber providing an air circulating space thereabout, a refractory trough extending along the bottom of said chamber, a burner at the forward end of the chamber positioned to direct a flame horizontally along the refractory trough, outlet passages for products of combustion at the forward end of the chamber at each side of the burner, whereby the general flow of the flame and products of combustion is in a loop from the trough up into said top portion, forwardly along the top portion and then downwardly at the forward end of the chamber at each side of the burner, the outflowing gases thus moving at right angles to the axis of the burner, and means for propelling air to be heated through the space between said chamber and said walls.

10. Apparatus of the class described comprising a combustion chamber having a bottom and sides,

with the sides formed oi heat-conducting material and onverging inwardly to form an overarched top portion, walls spaced from the exterior of said chamber providing an air-circulating space thereabout, the bottom oi. said chamber comprising a trough that extends along the chamher, a burner at theforward end 01' the chamber positioned to direct a flame horizontally along the trough, outlet passages for products or combustion at the forward end of the chamber at 

